The general principle of security components is described in FR 2509873. It describes a colored subtractive diffraction filter comprising an optical medium with a variable refraction index of thickness (t) and minimum periodicity (d) in relation to the wavelength of the incident light. The filter is subject to certain constraints with regard to: 1) the relative refraction indices of the internal structure and of its environment, 2) the relative values of the wavelength of the incident light in relation to the periodicity, and 3) the relative refraction indices of the optical medium and of its environment.
That filter produces at the same time color subtraction reflection spectra sensitive to angles and color subtraction transmission spectra conforming to these physical parameters. Such a component produces a variable colored effect as a function of the lighting and viewing directions. The component has, e.g., a greenish appearance with certain orientations and a reddish appearance with other orientations for a given grating type. However, these color variations are subtle and sometimes difficult to detect with the naked eye (independently of machine readability).
The use of such a component for security marking therefore requires that persons who must verify the authenticity of the product or the document are able to verify in a clear manner without ambiguity that the anticipated effects were produced even under lighting conditions that are less than optimal.
Also known are components producing holographic effects such as U.S. Pat. No. 4,856,857 and U.S. Pat. No. 5,992,463, as well as EP 743192 and WO 01/07268.
Also known is GB 2,126,949. It describes an authentication component having an optical security component the high index layer of which is first coated with a low optical index layer and then by a contrast layer. That component produces color change effects as a function of orientation upon rotation in relation to an axis perpendicular to the surfaces. Nevertheless, these effects are sometimes difficult to discern.